Stereogenic properties of 1,3-disubstituted derivatives of cyclotriphosphazene: cis (meso) and trans (racemic) isomers

Abstract

The di-spiro derivatives of the reaction of cyclotriphosphazene with either 3-amino-1-propanol or N-methylethanolamine are expected to exist as cis and trans geometric isomers and be meso and racemic, respectively. The isomers were separated by column chromatography on silica gel and analysed by elemental analysis, mass spectrometry, and 31P and 1H NMR spectroscopies. X-ray crystallography was used to characterise both the cis and trans isomers of the N-methylethanolamino di-spiro derivatives and the cis isomer of the propanolamino di-spiro derivative but, due to a mirror plane for the trans isomer, it was not possible to determine uniquely the NH and O positions and, hence, the cis or trans configuration of the compound. The stereogenic properties of all the compounds were investigated by 31P NMR spectroscopy on addition of a chiral solvating agent (CSA), (S)-(+)-2,2,2-trifluoro-1-(9′-anthryl)ethanol. The CSA results confirm that both cis isomers are meso and the trans isomer of the N-methylethanolamino di-spiro derivative is a racemate, as expected. It was also shown that the second isomer of the propanolamino di-spiro derivative was a racemate, confirming that it is the trans isomer and, hence, confirming the original predictions of the stereogenicity of the 1,3-disubstituted cyclotriphosphazenes.